A three-dimensional measurement machine is a measurement instrument used in dimensional metrology that is designed to capture the coordinates of different points of a part to be measured in order for example to check conformity of the dimensions, geometry and/or shapes of the part.
Such machines generally include a base on which is mounted a measurement arm, at the end of which there is a measurement head. The measurement arm conventionally comprises rigid segments linked together for example using pivots and/or ball joints provided with encoders precisely measuring the movements of the segments in relation to one another. The measurement head has means for capturing a point on an object to be measured.
There are two main types of measurement head that are distinguished by whether the measurement is taken with or without contact with the part. With contact measurement heads, the measurement head has a probe that comes into contact with the part. The capturing of coordinates is commanded either automatically by detection of the contact between the probe and the surface of the part, or by the operator using a command button that the operator actuates when the probe is in contact with the desired zone of the part. With contactless measurement heads, the measurement head incorporates an optical sensor (scanner) generally comprising a laser diode pointer that illuminates a small zone of the surface of the part to be measured and a camera that captures the reflected light, obtaining the distance measurement thereof by calculating the delay between the light signal emitted and the light signal reflected by the surface of the part to be measured. The capture command may be given continuously or as required by the operator using a command button that the operator actuates when the sensor is in contact with the desired zone of the part.
When the capture command is given, the relative positions of the different segments of the measurement arm are saved in the memory of a processing unit linked to the measurement arm. The processing unit is set up to determine the coordinates of the measurement points in a reference system, generally attached to the base, on the basis of the dimensions of the segments, the relative positions thereof and measurement-head information. Correctly processing these coordinates enables the dimensions, shape and geometry of the part to be measured to be obtained.
The angular encoders attached to the ends of the rigid segments are for example point encoders. These encoders generally have a circular plate marked with several equidistant points all located on a single circle. During one rotation, a sensor (which may be magnetic or optical, depending on the nature of the points) counts the number of points that pass before the counting cell thereof, calculating therefrom the amplitude of the rotation effected. A measurement arm generally has four angular encoders:
a first encoder measures the rotation about a vertical axis of the first rigid segment of the measurement arm in relation to the rigid base;
a second encoder measures the rotation about a horizontal axis of the first rigid segment of the measurement arm in relation to the rigid base;
a third encoder measures the rotation about a horizontal axis of a second rigid segment of the measurement arm in relation to the first rigid segment of the measurement arm with which it is articulated;
a fourth encoder measures the rotation about a horizontal axis of the measurement head in relation to the second rigid segment of the measurement arm on which it is mounted.
There are numerous factors liable to affect the precision of a measurement, including:
axial and radial play in the axes of the bearings of the arm joints;
bending of the segments, which do not provide absolute rigidity;
the resolution of each encoder.
The error generated by these different factors can increase with the amplitude of the movements of the different elements of the measurement arm. In general, the greater the variation in the position of each of the elements making up the arm (segments, joints, encoders, etc.), the greater the negative effect of the measurement error.
Thus, when moving from one measurement point to another, imprecisions from all sources are accumulated, resulting in an overall imprecision in the determination of the coordinates of the measurement points and therefore of the geometric feature it is intended to measure, such as the difference between measurement points.